Electrode for an electrolytic cell, use thereof and method using same

ABSTRACT

The present invention relates to an electrode preferably an insoluble electrode for an electrolytic cell. The electrode is located within an enclosure defining a chamber, a wall of said enclosure being formed by a membrane allowing ions to pass therethrough. The enclosure has an opening for feeding electrolyte, an opening for evacuating electrolyte and means conducting the upward current of electrolyte with a velocity in the vicinity of the electrode of at least 0.01 m/s. The invention relates also to plants and processes using such electrode for the plating or deplating of metal strips.

FIELD OF THE INVENTION

This is a national stage application of PCT/8392/00022 FILED May 27,1992.

The present invention relates to an electrode, preferably an insolubleelectrode for an electrolytic cell. The invention relates also to plantsand processes using such electrode for the plating or deplating of metalstrips.

BACKGROUND OF THE INVENTION

Insoluble electrodes are commonly employed in processes for coatingstrips of metals by electrochemical routes, preferably of zinc-coated orgalvanized steel strips, with the aid of metals or metal alloys, inaccordance with which an electrolyte laden with salts of the coatingmetals is recycled between the cathodic metal strip to be coated and theinsoluble anode.

Depending on the electrolyte used, for example sulphate- orchloride-based electrolytes, the use of this process generates gases atthe anode, for example oxygen or chlorine, which partially undergoundesirable bonding with the coating metals or which, because of theirhigh reactivity or their toxicity, have detrimental consequences duringthe use of the coating process or for the environment. These gases whicharise at the anode mix with the electrolyte and can, consequently, causeundesired reactions and enter the environment, given that during theelectrolytic coating of metal strips the electrolyte circuit on thestrip cannot be separated from the atmosphere.

A process for coating galvanized steel strips with the aid of ironcompounds or of alloys of the latter is known. To do this, asulphate-based electrolyte laden with salts of coating metal isconducted in a closed circuit between the steel strip to be coatedcirculating endlessly and the insoluble anodes. Because of knownelectrochemical processes, iron precipitates in the form of ironcompounds at the cathodic metal strip. Divalent oxygen is released atthe anode, oxygen which comes into contact with the metal salts, moreparticularly because of the recycling of the electrolyte. This oxygenoxidizes a part of the divalent iron to trivalent iron, with the resultthat large quantities of iron oxide are produced, which soil theelectrolytes and which must be separated from the circuit by making useof costly filtration processes.

Furthermore, the formation of Fe³⁺ reduces the cathodic efficiency ofthe current and deteriorates the adhesion of the deposited layer.

Finally, the use of salts of the coating metal, or the use of iron incorresponding dissolving plants and the replacement of other substancesemployed which are entrained together increase the cost of such acoating process very appreciably.

BRIEF DESCRIPTION OF THE INVENTION

To solve these problems, the applicants have developed a specialelectrode which is particularly useful in processes for electrochemicalcoating of metal strips, but which is also suitable in other processessuch as processes for electrochemically removing a coating from a stripsuch as a steel strip.

The electrode according to the invention is placed in an enclosuredefining a chamber and one wall of which is formed by a membraneallowing ions to pass through the latter, the said enclosure having afirst opening for feeding the chamber with an electrolyte and a secondopening for removing the electrolyte from the chamber.

The enclosure or the electrode is advantageously provided with meansintended to ensure a minimum velocity of the electrolyte in the vicinityof the electrode, a velocity which is preferably higher than 0.1 m/s, inparticular higher than 0.5 m/s.

Such means are, for example, baffles or fins directing the flow ofelectrolyte in the chamber or a part thereof.

In one embodiment, the baffles or fins extend from the vicinity of thefirst opening of the enclosure as far as the vicinity of the secondopening of the enclosure, so as advantageously to divide the chamberinto a number of separate compartments extending between the electrodeand one wall of the chamber or enclosure, in particular the membrane.

In another embodiment, the said baffles or fins create an at leastpartially upward current of electrolyte in the vicinity of theelectrode. According to a special feature of this embodiment, thebaffles or fins extend in a substantially vertical direction from thevicinity of the lower part of the enclosure as far as the vicinity ofthe upper part of the enclosure, so as to define channels conducting theelectrolyte into the upper part of the enclosure; this part having anopening for drawing gases out of the chamber and an opening fordischarging the electrolyte.

The baffles or fins advantageously extend at least from one edge of theelectrode as far as the opposite edge of the latter.

The membrane is preferably an anionic or anion exchange membrane or acationic or cation exchange membrane. On the outer side of theenclosure, it is advantageously provided with a protective layer or webmade, for example, of synthetic material (polymer, polyester, etc.)advantageously reinforced with fibres (glass).

A porous support preferably extends in the vicinity of the membrane andis used to support at least a part of the latter. Such a support is, forexample, a perforated component, a porous web or a trellisadvantageously made of Zr, Ti or of stainless steel.

In one embodiment, the support has a layer acting as an electrode on theface opposite that adjacent to the membrane, whereas according toanother embodiment, the membrane rests on a support acting as anelectrode, the said support being provided with an insulating layer onits face adjacent to the membrane.

The membrane of an electrode according to the invention advantageouslyhas a thickness of between 50 and 150μ. In the case of an anionicmembrane, it preferably has a multilayer structure where at least onelayer is obtained by grafting an amino monomer or a precursor of anamino compound onto a polymer substrate and by cross-linking.

Another subject of the present invention is the use of an electrodeaccording to the invention in an electrolytic cell.

Finally, another of its subjects is a process for electrochemicalcoating of galvanized steel strips by means of metals or metal alloy. Inthis process, an electrolyte laden with salts of the coating metals isrecycled in a known manner between the metal strip to be coated(cathode) and the insoluble anode. According to the process, inaccordance with the invention, an electrode in accordance with theinvention is employed as insoluble anode. The membrane is arrangedbetween the anode and the metal strip to be coated so as to form aseparation between a cathodic space adjacent to the strip and the anodechamber defined by the anode enclosure. In the process, in accordancewith the invention, a first primary electrolyte circuit is created inthe chamber and a second secondary electrolyte circuit in the cathodicspace, the membrane preventing the transfer of gases generated at theanode into the second electrolyte circuit and the transfer of salts ofthe coating metals from the cathodic space towards the first electrolytecircuit. In this case the gases remain in the electrolyte circuit whichis maintained separately in the anodic space and can be regularlyremoved. The electrolyte of the anodic circuit is not laden with thecoating metals. The gas which is always formed can be removed from thiscircuit in a relatively simple manner. The two circuits are obviouslyseparated from one another, with the result that mixtures cannot form.

By using the electrode according to the invention, processes have beenproposed in accordance with the invention for coating metal strips,preferably galvanized steel strips with the aid of iron, or of ironcompounds or of alloy containing iron. Depending on the nature of theelectrolyte employed in the cathodic space or vessel, it is proposed touse, as diaphragms, cation exchange membranes or anion exchangemembranes which are known per se. What is known as bipolar membranes canalso be used with a corresponding modification or adaptation of theelectrolytes.

If an anion exchange membrane of suitable nature is arranged between theanode and the metal strip to be coated, when a sulphuric electrolyteenriched in iron and zinc sulphate is employed in the cathodic space,then only the transfer of SO₄ ⁻⁻ ions into the anode chamber is ensuredas charge transport and the transfer of the salts of coating metals isprevented. The electrolyte devoid of metal and composed of water andsulphuric acid in the anode chamber is here enriched in sulphuric acid.The oxygen which forms at the insoluble anode can be removed from theanode chamber. The transfer of oxygen into the cathodic space isprevented by the corresponding anion exchange membrane.

When a cation exchange membrane is employed for the plating of metalstrips with iron, compounds or alloys containing iron and when asulphuric electrolyte is employed in the cathodic space, the chargetransport takes place by the transfer of hydrogen ions from the anodechamber into the cathodic space. The oxygen which forms at the anode isremoved from the sulphuric electrolyte devoid of iron from the anodiccircuit. The transfer of oxygen into the cathodic space is alsoprevented by this cation exchange membrane.

When a chloride-containing electrolyte enriched in iron or zinc chlorideis employed in the cathodic space, it is also possible, in accordancewith the present invention, to employ appropriate anion exchangemembranes. When this process is used, chlorine ions are allowed to enterthe anode chamber as charge carriers. However, the transfer of metalsalts into the anodic space is prevented. The electrolyte, which iscomposed of water and hydrochloric acid in the anode chamber is enrichedin chlorine ions released in the form of gas at the anode and isadvantageously removed in a controlled manner with the electrolytecircuit from the anode chamber. A transfer of chlorine into the cathodicspace is prevented by the appropriate exchange membrane.

When a chloride-containing electrolyte is employed in the cathodicspace, it is also possible to employ cation exchange membranes ofsuitable type. In this case, too, the transfer of acids and salts fromthe cathodic space into the anodic space is again prevented. The chargetransport takes place by the transfer of hydrogen ions from the anodicspace or chamber into the cathodic space. The gases separated at theanode are removed. A transfer of the separated gases into the cathodicspace is prevented by the cation exchange membrane.

By virtue of this process, in accordance with the present invention forcoating metal strips with iron, the formation of trivalent iron and ofiron oxide, which forms iron sludge in the electrolyte when the knownprocess is used, is completely prevented, since this sludge results froman oxidation due to the oxygen released at the anode.

Given that the action of atmospheric oxygen in the cathodic circuitcannot be completely prevented during the coating with iron performed inaccordance with the present invention, a certain quantity of trivalentiron is still also formed in the cathodic circuit. This trivalent ironsoils the cathodic circuit, with the result that this electrolyte mustalso still be filtered. In accordance with the present invention, it isconsequently proposed to feed the circuit with cathodic electrolyte,with a view to replacing the iron removed during the coating, with acorresponding proportion of iron feed, for example in an intermediatedissolving station. The necessary proportion of added elemental ironsuffices, because of the excess, to reduce the trivalent iron todivalent iron, with the result that iron oxide sludge is no longerformed in the cathodic electrolyte circuit.

The sulphuric acid which is enriched in excess during the use of asulphuric electrolyte in the cathodic circuit and during the use of ananion exchange membrane in the anodic circuit, is employed in thedissolving station and is thus returned into the cathodic circuit, wherethe rate of dissolving of the iron and of the other coating metals, forexample zinc, is considerably accelerated.

The gaseous chlorine which is formed at the anode during the use of achloride-containing electrolyte in the cathodic circuit and of an anionexchange membrane is removed by suction from the anodic circuit and isburnt to hydrochloric acid by the gaseous hydrogen formed in thedissolving station and is used to accelerate the dissolving of themetals and is consequently returned into the cathodic circuit via thedissolving station.

Another subject of the present invention is a process for removing alayer of metals or of a metal which is present on a metal strip such asa steel strip. This layer of metals or metal is, for example, a layerdeposited electrolytically, such as a protective layer of Zn or Znalloy. By way of particular example, the Zn or Zn alloy layer depositedas a protective layer on a face or strip has a thickness of between 0.1and 2 microns (preferably less than 1 micron). Such a layer ispreferably obtained by subjecting the strip to an electrolytic treatmentin a bath containing from 15 to 100 g/l, advantageously from 30 to 80g/l of Zn. The current density in the cells is, for example, between 20and 200 A/dm², but is preferably between 40 and 150 A/dm². The electrodein accordance with the invention can be advantageously employed for thisdeposition.

During this deposition, the strip and optionally the electrolyte in thesaid cells are set in motion in the cells. The relative velocity of thestrip in relation to the electrolyte is advantageously between 1 and 8m/s, preferably between 3 and 5 m/s.

In the process, in accordance with the invention, for removing a layerof a metal or of metal alloy from a strip, an electrolyte is recycledbetween the strip acting as the anode and an insoluble cathode; anadvantageously anionic membrane being arranged between the strip and thecathode, so as to form a separation between a cathodic space and ananodic space adjacent to the strip.

This membrane makes it possible to overcome the formation of a deposit(black in the case of Zn and of Ni) on the cathode by the metalsredissolved in the electrolyte, such as, for example Zn and/or Ni; thisdeposit reducing not only the efficiency of the cathode but, above all,the lifetime or the service life of the latter.

This membrane may be a porous web (pores of a few microns, 1 to 50μ),but is preferably an anionic membrane, that is to say, a membrane whichdoes not permit, or which limits, the passage of cations (such as Zn⁺⁺,Ni⁺⁺, Fe⁺⁺) through the latter.

In the case where an acidic electrolyte is employed, it has been noticedthat a release of hydrogen is present at the surface of the cathode. Toavoid hydrogen bubbles combining to form large bubbles, it has beennoticed that it is useful to employ the membrane as the wall of achamber adjacent to the cathode and to maintain a so-called secondaryelectrolyte stream or flow in the said chamber.

The velocity of the electrolyte in the chamber is, for example, higherthan 0.1 m/s, but is preferably lower than 1.5 m/s, to ensure that thehydrogen bubbles do not combine to form large bubbles.

The electrolyte circulating in the chamber adjacent to the cathode,called secondary electrolyte hereinafter, preferably has a compositiondiffering from the primary electrolyte; that is to say, the electrolytein contact with the strip. The secondary electrolyte is advantageouslyan electrolyte containing no Zn or Ni, but containing from 50 to 100 g/1of Na₂ SO₄ and whose pH is preferably adjusted to a value of 1.5 to 2.

The upper part of the chamber is also preferably subjected to a gassuction. For example, a vacuum is created in the upper part of thechamber such that the pressure in the chamber is lower than0.75×atmospheric pressure.

The primary electrolyte employed in the deplating cell may, for example,be an electrolyte containing less than 50 g/l of free acid,advantageously less than 5 g/l, preferably approximately 1 g/l of freeacid (for example free SO₄ ⁼). The pH of the electrolyte isadvantageously from 1.5 to 2.

The current density employed in the "deplating" cell (cell for removinga metallized layer) in which the cathode is placed in a chamber isadvantageously lower than 60 A/dm², but is preferably between 15 and 30A/dm² in the case of an acidic electrolyte.

The temperature of the primary and secondary electrolyte isadvantageously between 20° and 60° C., preferably between 40° and 60° C.

Other special features and details of the invention will emerge from thefollowing detailed description, in which reference is made to thedrawings appended hereto.

DESCRIPTION OF THE DRAWINGS

In these drawings:

FIGS. 1 to 5 show various embodiments of electrodes in accordance withthe invention,

FIGS. 6 and 7 show electrodes similar to those shown in FIG. 2, butemployed in an electro-deposition cell,

FIG. 8 is an elevation view with partial cutaway of a preferredembodiment of an electrode in accordance with the invention,

FIGS. 9 and 10 are views in section along the

lines IX--IX and X--X of the electrode shown in FIG. 8,

FIG. 11 shows, in perspective and on a larger

scale, a part of the strips of the electrode shown in FIG. 8,

FIG. 12 is a diagrammatic view of a plant employing electrodes inaccordance with the invention,

FIGS. 13 and 14 show, in section and on a larger scale, a steel stripwhich has been obtained by employing electrodes in accordance with theinvention, and

FIGS. 15 to 18 show particular embodiments of plant employing electrodesin accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows in perspective an electrode in accordance with theinvention, which is advantageously employed in a "deplating" cell, butwhich can also be employed for the deposition of Zn, Zn--Ni, Zn--Fe oranother Zn alloy.

This electrode comprises a support 75 carrying a plate 76 intended toform the anode or the cathode. The support 75 forms an enclosure whichhas a window in which a membrane 77 is placed.

The membrane 77 forms a wall of the enclosure which defines a chamber78, 79. This membrane allows ions such as anions or cations to passthrough.

The enclosure has a first opening 100 for feeding the chambers 78, 79with electrolyte and a second opening 101 for discharging electrolytefrom the chambers 78, 79.

To ensure a minimum velocity of the electrolyte in the vicinity of theelectrode 76 for removing gases (such as oxygen when the electrode worksas an anode, or hydrogen when the electrode works as a cathode in a"deplating cell" deplating process) formed in the vicinity of theelectrode, the enclosure is provided with a guiding wall or fin 102extending between the said first and second openings, so as to dividethe enclosure into two compartments 78, 79 which are adjacent butseparated from one another. The fin 102 extends between the electrode 76and the wall of the enclosure provided with the membrane.

By virtue of this fin 102, it has been possible to ensure an electrolytevelocity of at least 0.04 m/s in the compartments 78, 79 in the vicinityof all points of the electrode or plate 76. In the case shown in FIG. 1,the electrolyte was delivered to the compartments 78, 79 with a velocityof the order of 0.5 m/s. The electrode 76-membrane 77 distance was 0.5cm.

FIG. 2 shows in section another embodiment of an electrode in accordancewith the invention.

The electrode 80, made of titanium but provided with an active layer, isintegrally attached to a support 81 by means of arms 82.

The support 81 forms with a membrane 83 an enclosure surrounding theelectrode 80. This membrane 83 is secured to a grid or trellis 84 madeof titanium and is provided on its face opposite that adjacent to theelectrode with a porous film 85 protecting the membrane. This film isresistant to acids and is reinforced with fibers. This film is, forexample, a polyester film.

When such an electrode is employed in a "deplating" cell, the membraneis anionic. Such a membrane is, for example, of multilayer structure,each of the layers consisting of a membrane obtained by the processdescribed in FR-8900115 (Application No.). A membrane of this type isprepared by grafting an amino compound onto a polymer substrate(ethylene-co-poly-tetrafluoroethylene film) and by crosslinking thelatter.

During an operation of removal of undesired Ni deposited on the firstlayer of Zn, Zn⁺⁺ and Ni⁺⁺ ions leave the face of the strip facing thecathode. These cations cannot pass through the anionic membrane, withthe result that a rapid deposition of Zn, Ni on the cathode is avoided.This makes it possible to increase the lifetime or the service life ofthe electrode.

Hydrogen is released in the vicinity of the cathode in the enclosureformed by the support and the membrane, while SO₄ ⁼ anions pass throughthe membrane to leave the enclosure.

To remove the gas formed in the enclosure (hydrogen in the electrode ofthe "deplating" cell as described above), the latter has an opening 103.This opening 103 advantageously permits a communication of the chamber78 with a conduit 104 in which a suction system (vacuum pump, fan or thelike), not shown, is fitted.

To avoid the formation of large bubbles of gas (hydrogen) whichinterfere with correct functioning of the electrode, the enclosure isconnected to a device for circulating electrolyte in the enclosure andto a system for removing the gases; in particular, a system producing avacuum in the upper part of the chamber, this vacuum beingadvantageously such that the pressure in the upper part of the chamberis lower than 0.75×the atmospheric pressure.

The velocity of the electrolyte in the chamber was higher than 0.1 m/s,but is nevertheless preferably lower than 1.5 m/s. Such a velocity makesit possible to ensure that the gas bubbles (hydrogen in the presentcase) do not combine to form large bubbles perturbing a correctfunctioning of the electrode.

FIGS. 6 and 7 show an electrode similar to that shown in FIG. 2. It isemployed as a cathode for depositing Zn and Fe electrolytically on thesteel strip.

In the case of FIG. 6, the membrane 83 is a cationic membrane, with theresult that the SO₄ ⁻ anions formed in the vicinity of the strip 3remain in the primary electrolyte, while iron and zinc are deposited onthe strip. Oxygen (oxygen produced by the decomposition of water) isreleased at the cathode 80 and is removed by the conduit 104.

In the case of FIG. 7, the membrane 83 is an anionic membrane allowingthe SO₄ ⁼ anions formed in the vicinity of the strip 3 to pass towardsthe cathode 80. The oxygen released at the cathode is removed by theconduit 104.

In FIG. 3, the enclosure surrounding the electrode 80 is still formed bya support 81 and a membrane 83. The electrode consists of a trellis orperforated plate made of titanium or zirconium, provided with an activelayer 87 on the face facing the chamber 78 defined by the enclosure.

The membrane 83 is carried by the trellis and is coated with aprotective porous layer 85.

The electrode shown in FIG. 4 is similar to that shown in FIG. 5 exceptthat an insulating porous web 88 is placed between the trellis and themembrane.

FIG. 5 shows in section another embodiment of an electrode in accordancewith the invention.

This electrode 80 is adjacent to the membrane 83 which closes off thewindow of the enclosure. This enclosure defines a lower chamber 78 andhas an opening or passage 100 for delivering electrolyte into thechamber 78, an opening or passage 101 for discharging electrolyte out ofthe chamber 78 and an opening 103 for discharging gases formed in thechamber, in particular in the vicinity of the electrode 80. Theseopenings are situated at a level above the electrode 80 and the membrane83.

A fin 105 extends between two opposed walls of the enclosure (front wall106 provided with the membrane 80 and rear wall 107) so as to define achannel 108 for delivering the electrolyte entering the chamber 78through the passage 100 in the vicinity of the bottom 781 of thechamber. This channel 108 is extended by a distribution chamber 109adjacent to the bottom 781. This distribution chamber 109 has a wall 110which has a series of orifices 111 for distributing the electrolyte intoa series of channels 112 defined between the vertical fins 113. Thesefins 113 extend from the vicinity of the bottom 781 of the chamber ormore precisely from the wall 110 as far as the vicinity of the upperpart or more precisely up to a level B which is higher than but adjacentto the upper level A of the membrane 83 and of the electrode 80. Thefins 113 which therefore extend between at least two opposed edges120-121 of the electrode ensure an upward movement of the electrolytealong the electrode 80, such a movement (from the lower edge 120 towardsthe upper edge 121) promoting the removal of gas particles out of theelectrolyte towards the upper part of the chamber which isadvantageously subjected to a pressure reduction (suction of gas throughthe passage or opening 103).

In the direction of their width, the fins extend from the electrode 80as far as the rear wall 107 of the enclosure so as to define separatechannels 112 extending from the electrolyte distribution or divisionchamber 109 as far as the upper part of the enclosure.

FIG. 6 is a partially cutaway front view of an embodiment of anelectrode in accordance with the invention.

This electrode comprises a support 81 which has an electrolyte channel130 towards the lower part 131 of the electrode (arrow E) and anelectrolyte discharge channel 132 (arrow S) in the upper part 133 of theelectrode. The ends 135 of these channels 130, 132 form lugs used asmeans of securing and placing the electrode in an electrolytic cell. Thesupport 81 is made of a material which is insoluble in the electrolyteor is covered with a protective layer which is insoluble in theelectrolyte.

This support 81 is, for example, made of titanium.

A first frame which has two windows 137, 138 is applied against thesupport 81. This frame 136 has grooves in which seals made of syntheticmaterial are housed. This frame 136 is made, for example, of aninsulating synthetic material which is resistant to the electrolyte.

Along its lower 139 and upper 140 edges, the frame has a series ofchannels 141, 142 extending between the face of the frame which isapplied against the support 81 and the face opposite the said faceapplied against the support 81, so that the said channels 141, 142communicate with the delivery conduit 130 and with the discharge conduit132 respectively, via orifices 153 with which the said conduits 130, 132are provided.

The windows 137, 138 are separated from one another by a cross member144 which has a dish 145 on the face opposite that facing the support81. Channels or passages 146 are hollowed out in the said cross member144 so as to extend between an opening 147 adjacent to one edge of thecross member 144 and an opening 148 adjacent to the opposite edge of thesaid cross member 144, the said openings 147, 148 being situated on theface of the cross member opposite that facing the support 81.

Two titanium plates 149 are applied against this frame 136, withinterposition of leakproofing seals 150 housed in grooves with which thesaid plates are provided. Along their edges these plates are providedwith a protuberance 151 forming a kind of basin. These plates 149 areperforated along the protuberance in the vicinity of the lower and upperedges 152, 153 so as to form channels 154 situated in the extension ofthe channels 141 and the openings 147, 148 of the frame 136.

Each plate 149 also has two holes 155 intended to provide a passage fora cylindrical member 156 made of titanium or of another material whichis electrically conductive but resistant to the electrolyte. Thiscylindrical member is provided with a head 157 one wall of which isintended to bear against the plate 136 with interposition of circularseals 158 housed in the grooves of the member 156 or more precisely ofthe head 157 and of the plate 149 and of a seal 159 forming a sleevepartially covering the cylindrical member 156 and the face of the head157 facing the plate 149.

In the vicinity of its end opposite that carrying the head 157, thecylindrical member 156 has a tapped hole 160 intended to work with thethreaded shank 161 of a bolt 162. For each bolt, the support 81 has anorifice 163 intended to provide a passage for the shank 161. One end ofthe orifice 163 opens into a cavity 164 intended to receive the head 165of the bolt 162, while the other end of the orifice opens into a hollow166 with which the support 81 is provided, the said hollow being usedfor the correct placing of the cylindrical member in relation to thesupport 81.

When the bolt 162 is tightened, the cylindrical member 156 is pressedagainst the support 81 so as to ensure leakproofing between the support81, the frame 136, the plate 149 and the head 157 of the member 156.

The plate 149 carries a layer 167 made of synthetic material which iselectrically insulating and whose thickness is such that the face 168 ofthe free end of the head 157 and the face 169 of the layer 167 oppositethat bearing on the plate 149 both lie substantially in the same plane.The latter corresponds to the plane along which the titanium electrode170 lies. By virtue of the insulating layer 167 and the insulatingsleeve 159 it is possible to ensure the insulation of the plate 149 inrelation to the current delivered through the bolt 162 and thecylindrical member 156 to the electrode 170.

This electrode 170 consists of a series of vertical titanium strips 171joined together by rods or other electrically conductive carrier (plate)members 173. These strips are advantageously parallel to one another.However, these strips could have been slightly inclined in relation toone another. In this case, the lengthwise edges (172) of the stripsadvantageously must not touch each other.

These strips 171 and carrier members 173 are advantageously providedwith an electrically conductive layer.

The strips advantageously have a height h of 5 to 10 mm and areadvantageously separated from one another by a distance of between 5 and10 mm.

The strips, therefore, form between them a series of vertical channels11 intended to direct the electrolyte in the vicinity of the electrodeand in particular to ensure a minimum upward velocity of the electrolytein relation to the electrode (see FIG. 11).

The strips 171 or preferably the carrier members 73 are welded to thehead 157 to ensure an electrical contact between the electrode and theconductor bolt 162. It is obvious that other methods of securing thestrips in relation to the head 157 permitting an electrical contact arepossible.

The strips or fins 171 of an electrode extend in the vertical directionfrom the level N of the channels 154 adjacent to the lower edge 152 ofthe plate 149 up to the level M of the channels 154 adjacent to theupper edge 153 of the plate 149. The length l of the strips correspondssubstantially to the width L of the insulating layer 167.

A protective insulating porous web 88 which is covered with a membrane83 extends above the lengthwise edges 172 of the strips, edges which areopposite the lengthwise edges facing towards the head 157. This membrane83 is, for example, an anionic or cationic membrane when the electrodeis employed for depositing a metal or a metal alloy on a strip, but ispreferably an anionic membrane when the electrode is employed forremoving a deposit of a metal or of a metal alloy from a strip.

This web 88 and this membrane 83 are stretched between the protuberances151 so as to form a chamber 78 in which the electrode lies. A secondaryelectrolyte e2 can pass through the said chamber 78, this electrolyte e2being advantageously different from the primary electrolyte e1 adjacentto the strip 3 to be coated or to be treated in order to remove a layerof metal or metal alloy therefrom.

The free edges of the web 88 and of the membrane 83 are applied againsta face 174 of the protuberance 151, a face advantageously forming theouter side face of the protuberance 151 of the plate 149.

Along their edges, the membrane 83 and the web are pressed against, onthe one hand, a frame 175 of L-shaped cross-section and, on the otherhand, the protuberance 151 and a seal 176 fitted in a groove with whichthe protuberance 151, is provided. To hold the frame 175 against theprotuberance 151 U-shaped clamping sections 177 are employed.

One arm 178 of the section bears on the face of the protuberance 151which faces the support 81, while the other arm 179 of the section bearsagainst the frame 175, with the result that the latter 175; the web 88and the membrane 83 are clamped between the protuberance 151 and the arm179.

Four sections 177 are advantageously employed per plate 149, so as toclamp and to secure the web 88 and the membrane 83 substantially allalong the protuberance 151 of the plate 149. In a particular embodiment,the sections 177 have miter-shaped ends so that these four sections of aplate form substantially a continuous frame extending along theprotuberance 151 of the plate 149. The dish 145 of the cross member 144of the frame 136 makes it possible to lay down the clamping sections177, in the case of the protuberances 151 adjacent to the said crossmember. In the case of these protuberances, the arm 178 extends betweenthe face of the protuberance facing towards the support 81 and thebottom of the dish. A seal 180 made of synthetic material is insertedbetween the clamping sections 177 so as to prevent the primaryelectrolyte e1 from reentering the dish 145, but above all so as to forma beading 181 extending beyond the vertical plane in which the membranes83 lie and the vertical plane in which the arms 179 of the sections 177lie. Such a beading makes it possible to reduce, or even to completelyavoid, any risk of contact of the strip to be treated with a membrane.This makes it possible to increase the lifetime of a membrane.

The system for securing the membrane which is shown (sections 177)permits a rapid fitting or replacement of the membrane and also permits,if need be, easy maintenance of the electrode.

It is obvious that other systems for securing the membrane could havebeen employed.

The circuit for the secondary electrolyte e2 in the electrode shown inFIG. 6 will be described below:

The electrolyte e2 enters through the opening 100 and is delivered bythe conduit 130 in the vicinity of the bottom of the electrode (arrowE). The electrolyte e2 then passes through the channels 141 and 154 intothe chamber 78 in which it flows vertically, from the bottom upwards,between the strips 171 of the electrode.

The electrolyte leaves this chamber 78 through the channels 154 and 141which are adjacent to the upper edge 153 of the plate 149, to bedelivered, via the channel 146 pierced in the cross member 144 and thechannels 154 and 141 which are adjacent to the lower edge of the plate149 shutting off the window 137, into the chamber 79. The electrolytethen moves into the passages formed between the strips 171 of theelectrode in order finally to emerge from the chamber 79 via thechannels 154 and 142 which are adjacent to the upper edge 153 of theplate 149. This electrolyte is finally discharged from the electrode viathe conduit 132.

FIG. 12 shows diagrammatically a plant employing electrodes inaccordance with the invention.

This plant comprises:

a series of electrolysis cells 1 for depositing an Ni--Zn layer on theface 2 of a steel strip 3; these cells contain anodes in accordance withthe invention;

means 5 for providing the faces 2, 4 of the steel strip with a firstlayer of Zn before the steel strip is introduced into or immersed in thecells 1, so as to make it possible to remove chemically orelectrochemically the Ni deposited on the first Zn layer of the face 4,and

a plant 21 for removing the nickel deposited on the first Zn layer ofthe face 4, and for at least partially removing the said first layer.

The electrolysis cells 1 for the deposition of a Zn--Ni layer are, forexample, of the type described in DE-A-3510592, but comprising anodes inaccordance with the invention.

These cells 1 are connected to a storage vessel 54 by means of pumps, ofa feed conduit 58 and of a discharge conduit 59 so as to ensure asubstantially constant concentration of Ni and Zn in the electrolyte.The Ni and Zn concentration of the electrolyte is, for example, thatgiven in BE-A-881635 and BE-A-882525. The electrolyte may also containadditives such as polymers, ZrSO₄, etc.

The storage vessel 54 is connected to a device for enriching theelectrolyte in Zn and/or in Ni, so as to keep the Zn and Niconcentration of the electrolyte at a substantially constant value.

The means 5 for providing the faces 2, 4 of the steel strip 3 with afirst Zn layer preferably comprise electrolytic cells 11 into which thesteel strip 3 is introduced. These cells are also advantageously of thetype described in DE-A-3510592, but comprising anodes in accordance withthe invention.

The steel strip travels in the plant while bearing on the rollers 13, 14and on the return rollers 15.

The cells 11 contain an electrolyte (a solution of ZnSO₄) and areconnected to a storage vessel 16 by means of pumps 17, 18, of a feedconduit 19 and of a discharge conduit 20 for ensuring a more or lessconstant Zn concentration in the electrolyte. This storage vessel isconnected to a reactor (not shown) for enriching the electrolyte in Zn.

The plant 21 for removing the Ni which may be deposited on the Zn layerand for at least partially removing the said Zn layer consists, in theembodiment shown, of a deplating cell 50 advantageously comprising acathode in accordance with the invention.

After this deplating operation (removal of a metal layer), the strip 3is subjected to a rinsing by virtue of the rinsing device 51, to abrushing in a brushing plant 52 to ensure that all the Ni deposited onthe first Zn layer of the face 4 has been removed, and advantageously toa polishing in the unit 91.

The plant advantageously additionally comprises a series of storagevessels 54, 55, 56, 57. The first storage vessel 54 contains theelectrolyte intended to be delivered to the cells I via conduits 58fitted with pumps, while the second storage vessel 55 is intended tocollect the electrolyte leaving the electrolytic cells 1 via conduits59. The third storage vessel 56 contains the electrolyte intended to bedelivered to the "deplating" cell 50 via the conduit 60, while thefourth storage vessel 57 is intended to collect the electrolyte leavingthe "deplating" cell 50 via the conduit 61. A filter 72 is fitted in theconduit 63 to recover the Ni in the form of powder which has beenremoved from the steel strip. This Ni powder must be taken out of theelectrolyte because it is in a form which is difficult to dissolve.

A proportion of the electrolyte from the second storage vessel 55 andthe electrolyte from the fourth storage vessel 57 are conveyed viaconduits 62, 63 towards a plant 64 for regenerating or enriching theelectrolyte, the enriched electrolyte being next conveyed via a conduit65 towards the storage vessel 54 intended for feeding the cells 1.

Another part of the electrolyte from the second storage vessel 55 isconveyed via a conduit 66 towards the storage vessel 56 intended to feedthe "deplating" cell 50.

The plant additionally comprises a unit for storing and/or preparing 67secondary electrolyte; this electrolyte, lean in Zn and Ni, beingconveyed into the enclosure in which the cathode 53 is placed. This unit67 comprises a storage tank 68 connected by a conduit 69 intended todeliver electrolyte into the enclosure 53 and by a conduit 70 intendedto remove the electrolyte out of the enclosure and to return it into thetank 68. Water and sulphuric acid are delivered to this unit tocompensate for the losses of H₂ O and H₂ SO₄ (SO₄ ⁻ ) in the electrodechambers.

Electrolyte which is lean in Zn and Ni could optionally be conveyed intothe storage vessel 56 by a conduit.

In this plant, the steel strip has been provided with a first Zn layerwith a thickness of 1 micron. To obtain such a layer on the faces 2, 4of the strip, the strip was immersed in an electrolytic cell 11 in whichthe electrolyte contained 60 g/l of Zn. The current density between thecathode (the steel strip) and the anode 26 was 100 A/dm². The relativevelocity of the strip in relation to the electrolyte was 1.5 m/s.

Once the strip had been provided with the Zn layer, the strip wasdelivered into electrolytic cells 1 in order to deposit a layer ofZn--Ni on the face 2 of the strip.

In a particular embodiment of the plant shown in FIG. 12, a fine layerof Zn--Ni was deposited in the cells 11 on the two faces of the steelstrip 3. The thickness of the said layer was 0.5μ (weight per unit area:±3.5 g/m²), while the Ni content of the said layer was of the order of10%. To perform this deposition, the electrolyte employed was theelectrolyte employed in the cells 1.

The electrolyte which was employed in the cells 1 contained 25 g/l Zn⁺⁺,50 g/l Ni⁺⁺ and 75 g/l Na₂ SO₄. The pH of this electrolyte was 1.65 at57.5° C. The anode-steel strip distance was approximately 15 mm.

The primary electrolyte employed in the "deplating" cell in the testswhich were performed had the same composition as the electrolyte in thecells 1. However, it would have been possible to employ an electrolytecontaining less Zn⁺⁺ and Ni⁺.

The secondary electrolyte conveyed into the enclosure contained 75 g/lNa₂ SO₄ (pH of approximately 1.7).

The cathode-strip distance in the "deplating" cell was 16 mm. Thevelocity of the secondary electrolyte in the enclosure was 0.4 m/s,while the velocity of the primary electrolyte was 1.5 m/s.

Tests were performed with the "deplating" cell to remove a layer of Znor of Zn--Ni deposited electrolytically.

In these tests, the enclosure of the cathode had an anionic membrane of150μ thickness, sold by Morgane (France), while the current density inthe "deplating" cell varied between 0 and 50 A/dm².

When the current density was nil, no removal of Ni was observed. Next,the current density was increased, and an increasingly complete removalof Ni and of Zn was observed, as shown in the following table.

                  TABLE                                                           ______________________________________                                        Weight per unit area                                                          before passing through                                                        the "deplating" cell                                                                            Current density in                                                                         Zn + Ni/Fe or                                  g/m.sup.2         the "deplating"                                                                            Zn/Fe ratio of                                 Zn + Ni                                                                              flash Zn   cell         the face after                                 deposit                                                                              deposit    A/dm.sup.2   brushing                                       ______________________________________                                        3                  0           30-42                                          3                 15           16-17                                          3                 20           0-8                                            3                 25           0-5                                            3                 50           0-4                                                   3          20           0                                              ______________________________________                                    

The time of travel of the strip in front of the cathodes was 4 seconds.It is obvious that by employing a longer time of travel it is possible,while employing a density of 20-25 A/dm², to obtain an Ni+Zn/Fe ratioclose to 0 or equal to zero.

The plant shown, which makes it possible partially or completely toremove the Ni deposited on a Zn layer is a plant which makes it possibleto reduce the losses of electrolyte as much as possible by virtue ofrecirculation system. This also makes it possible to reduce the totalusage of Zn and Ni in the plant and to reduce the operating and capitalcosts of plants for purifying discharges.

It is obvious that the rinsing device can be equipped with a unit (notshown) for recovering electrolyte, Zn and Ni.

So as to reduce the losses of electrolyte further and to simplify theplant operation, a thin 0.5μ) layer of Zn--Ni is advantageouslydeposited in the cells 11. To perform such a deposition, the electrolyteemployed is advantageously the same as that employed in the cells 1. Inthis case, the same single electrolyte can be employed in the cells 1,11 and the "deplating" cells (cells for removing Ni and/or Zn and/or aZn alloy).

Similarly, to reduce the number of electrodes of different type employedin the plant, an electrode with a membrane is employed both in the"deplating" cells and in the cells for depositing a layer of Zn or of aZn alloy.

In the case of "deplating" cells, the current density is advantageouslylower than 60 A/dm². However, in the case of cells for depositing alayer of Zn, Zn--Ni or other Zn alloy, this density may be higher than60 A/dm², for example 100 A/dm².

Finally, FIGS. 13 and 14 show in section and on a larger scale,respectively, a steel strip which has been obtained in a plant of thetype shown in FIG. 12 and a steel strip one face of which has beensubjected to an overpickling or a polishing.

The steel strip 200 according to the invention is provided with anNi--Zn layer on one face. On the other face of the strip the remainingZn concentration is lower than 50 μg/m² (in particular than 10 μg/m²).This Zn remaining on this face is distributed uniformly andhomogeneously.

Such a distribution, combined with the presence of a very small quantityof Zn and Ni (advantageously less than 25 μg/m² and preferably less than10 μg/m²) makes it possible to obtain good phosphating.

A strip in accordance with the invention is, therefore, a strip whichhas one face covered with a layer of Zn--Ni and whose other face isprovided with Zn and/or Ni which are distributed uniformly and/orhomogeneously where the weight of Zn and/or Ni per unit area of the saidother face is greater than 0.1 μg/m² but smaller than 25, preferablythan 10 μg/m². A weight per unit area of 0.1 μg/m² is a weight per unitarea demonstrating the absence of an overpickling and therefore of theattack on a face of the steel strip.

A strip which it is possible to obtain by a process in accordance withthe invention has a face not covered with Zn and Ni, and has a roughnesswhich is substantially equal to that which the steel strip had beforeits treatment (deposition of a Zn--Ni layer).

Thus, it is possible to obtain a steel strip 200 which has an upper face201 and an upper face 202 of substantially equal roughness, one (201) ofthe said faces being covered with a layer of Zn--Ni 203.

When the strip has been subjected to an overpickling or to a polishing,the face 205 not covered with the Zn--Ni layer has undergone an attackmodifying the roughness of the steel strip. Moreover, an overpicklingwill cause a decrease in the thickness of the Zn--Ni layer 204, whileduring a polishing operation scratches will be formed in the steelstrip.

The steel strip in accordance with the invention can next be subjectedto a phosphating and be covered with one or more layers of paint on theface 105 which is not covered with the Zn--Ni layer. It has been notedthat it is possible to obtain a better adherence of the layers of paintor at least an adherence equivalent to that of a steel strip notprovided with a Zn--Ni layer.

To coat a strip electrochemically with a layer of a metal, it ispossible to employ an electrode with both an anionic membrane and acationic membrane. However, since an electrode with an anionic membraneis preferably employed for removing a layer of a metal, it may beadvantageous to employ the same electrodes with an anionic membrane bothfor the electrolytic deposition and for the electrolytic removal of ametal layer, so as to make it possible to employ an electrode once forthe electrolytic deposition and once for the electrolytic removal of alayer.

FIG. 15 shows, diagrammatically, a plant comprising, on the one hand, acell 1 for depositing a Zn--Ni layer on the face 2 of a galvanized strip3 and, on the other hand, a cell 50 for removing the galvanized layerfrom the face 4 of the strip 3. In this plant, the electrodes employedare electrodes in accordance with the invention, provided with anionicmembranes.

The electrolyte which leaves the electrode chamber 501 of the cell 50 isdepleted in SO₄ ⁼. This electrolyte is delivered by the conduit 502 intothe tank 503. Electrolyte leaving this tank 503 is conveyed via theconduit 504 and the pump 505 into the anode chamber 401 of the cell 1.When it passes through the anode chamber, the electrolyte becomesenriched in H₂ SO₄. This enriched electrolyte is delivered by theconduit 506 into a tank 507.

The tanks 503 and 507 are advantageously used in combination with a unit530 for compensating the losses of water and/or SO₄ ⁼ of the secondarycircuit of electrolyte in the electrodes. Such a unit comprises a mixingtank 531 for electrolyte originating from the conduit 532 from the tank507 and water and/or H₂ SO₄ originating from a conduit 510.

In this case, the case shown in FIG. 14, the electrolyte from the tank531 is delivered into the cathode chamber 501 by the conduit 508 inwhich the pump 509 is fitted.

The plant additionally comprises,

a storage vessel 511 for collecting the electrolyte leaving the cell 50;

a storage vessel 512 for collecting the electrolyte leaving the cell 1,an electrolyte lean in Zn--Ni;

a storage vessel 513 for feeding the cell 50 with an electrolyte lean inZn--Ni, and

a storage vessel 514 for feeding the cell 1 with an electrolyte rich inZn--Ni.

The storage vessel 514 receives via the conduits 515 and 516 electrolytefrom the storage vessels 511 and 512 and optionally via the conduit 517electrolyte originating from the tank 507. This conduit 517 optionallymakes it possible to purge the secondary circuit. The enrichment of theelectrolyte in the storage tank 514 is carried out by adding Zn--Nimetal powders and optionally H₂ SO₄ acid.

The enriched electrolyte is conveyed into the cell 1 by the conduit 518and the pump 519.

The storage vessel 513 which feeds the cell 50 with electrolyte lean inZn--Ni is fed with the electrolyte originating from the storage vessel512 and advantageously from the storage vessel 507 (conduit 520, pump 22and conduit 521, pump 523).

Such a plant makes it possible to reduce considerably the losses ofZn--Ni and permits a better utilization of the electrolytes.

Finally, FIGS. 16 to 18 show, diagrammatically, embodiments of a plantsimilar to that shown in FIG. 2.

In the form shown in FIG. 16, electrodes provided with an anionicmembrane are employed as anode in the cells 1 for the electrolyticdeposition of Zn or Zn--Ni on the face 2 of the strip 3 and as cathodein the cells 50 for removing a layer of Fe--Zn, Zn or Zn--Ni optionallycoated with Ni or Ni--Zn, a layer present on the face 4 of the strip 3.

The secondary electrolyte conveyed into the electrodes originates fromthe tank 68 of a unit for preparing electrolyte, which is fed with waterto obtain a correct dosage of the secondary electrolyte.

Secondary electrolyte may be conveyed via the conduit 71 towards thestorage vessels 55 and 56 which are intended to collect the primaryelectrolyte originating from the cells 1 and 50 respectively.

A proportion of the electrolyte originating from the tank 57, afterfiltration (filter 72), is returned to the tank 56 feeding the cell 50(conduit 90).

The other members, conduits and components of the plant shown in FIG. 16are similar to the members, conduits or components of the plant shown inFIG. 12. These same members, conduits and components are indicated bythe same reference numbers.

In this embodiment, it is possible, at the same time, to ensure amaterial balance equilibrium of the cells 1 (electrolytic deposition)and of the cell 50 (electrolytic removal) by virtue of the transfer ofelectrolyte from the tank 57 towards the tank 56 by the conduit 90,advantageously after filtration (filter 72).

The plants shown in FIGS. 17 and 18 relate to plants for the depositionof a first layer of Zn, ZnNi or other Fe alloys and of a second layer ofFe, Zn--Fe or iron alloy.

These plants permit, inter alia, the deposition of Zn or Zn--Ni on oneface 2 of the strip 3 and the deposition of Fe or Fe--Zn or other Fealloy on the face 4 of the strip 3; this face 4 being opposite the face2. The deposit of Fe or other Fe alloy (Fe--Zn) is intended to cover theZn or Zn--Ni which would have been deposited on the face 4. This depositof Fe or Fe alloy permits a phosphating of the face 4 as well as goodadherence of a layer of paint.

The plant in FIG. 17 comprises cells 600 and 601 with anodes which havean anionic membrane according to the invention. These are intended forthe deposition of a layer of metal on the face 2 of the strip 3. It isobvious that the cells could have included anodes arranged on both sidesof the strip so as to provide both sides of the strip with a layer ofmetal.

The plant comprises:

a storage vessel 602 for feeding the cells 600 via the conduit 603 withelectrolyte rich in Zn, Zn--Ni or other alloy;

a storage vessel 604 for collecting the depleted electrolyte leaving thecells 600 via the conduit 605;

a unit 606 for enriching the electrolyte originating from the conduit607 of the storage vessel 604, this enriched electrolyte being conveyedby the conduit 608 towards the storage vessel 602;

a storage vessel 618 for feeding the cell 601 via the conduit 609 withelectrolyte rich in Zn Fe, or other alloy;

a storage vessel 610 for receiving the depleted electrolyte leaving thecell 601 via the conduit 611;

a unit 612 for enriching the electrolyte originating from the conduit613 of the storage vessel 610; this enriched electrolyte being returnedvia the conduit 614 into the storage vessel 618, and

a unit for storing and preparing 67 electrolyte intended to circulate inthe anode chambers.

This unit 67 comprises a tank 68 connected by conduits 69 and 70 to theanodes to deliver secondary electrolyte and for returning the secondaryelectrolyte to the tank 68 after it has passed through the anodes.

This unit 67 comprises a water delivery 615 for compensating the lossesof water from the electrolyte or the increase in its H₂ SO₄ content. Thesurplus of H₂ SO₄ in the electrolyte, due to the latter's passagethrough the anodes is advantageously conveyed via the conduit 616 intothe storage vessels 604 and 610 to receive the depleted primaryelectrolytes leaving the cells 600 and 601.

Advantageously, only a proportion of the electrolyte from the tanks 604and 610 is conveyed towards the enrichment units 606 and 612. In thiscase, conduits 630 and 631 allow electrolyte to be conveyed directlyfrom the tanks 604, 610 towards the storage vessels 602 and 618.

FIG. 18 shows a plant similar to that shown in FIG. 16, except that thecells 600, 601 included anodes provided with a cationic membrane, andconsequently, the secondary electrolyte is not conveyed into the storagevessels 604 and 610 after passing through the anode chambers.

In FIGS. 17 and 18, the same reference signs denote identical members.

The storage vessels for feeding the cells with electrolyte rich in, forexample, Zn, Ni, the storage vessels for receiving the depletedelectrolyte leaving the cells and the units for enriching theelectrolyte are advantageously of the type described in applicationEP-A-0388386.

As can be seen from FIGS. 15 to 18, the chamber of an electrode of afirst cell (1, 600) and the chamber of an electrode of a second cell(50, 601) are mounted in one and the same circuit.

In an embodiment, particularly when the membranes employed are anionic,the electrolyte circuit is such that electrolyte leaving the chamber ofan electrode of a first cell (1) is conveyed, optionally after treatment(addition of water, H₂ SO₄, etc.) into the chamber of an electrode of asecond cell (50) and electrolyte leaving the chamber of an electrode ofa second cell is conveyed into the chamber of an electrode of the firstcell, optionally after treatment (addition of water, etc.).

We claim:
 1. Process for the electrochemical plating of a metal stripwith a metal in a cell provided with an insoluble anode and in which themetal strip acts as a cathode, in which a first electrolyte containing asalt of a plating metal is recycled between a cathodic metal strip to beplated and an insoluble anode, in which the insoluble anode is anelectrode, wherein said electrode comprises a set of vertical finshaving a bottom and a top, said set of fins being located within anenclosure defining a chamber, wherein a vertical wall of said enclosureis formed by a membrane allowing ions to pass therethrough, saidenclosure having a feeding passage for feeding a second electrolyte intothe chamber at the bottom of the set of fins and an outlet passage forevacuating electrolyte from the chamber at the top of the set of finscreating an upward current of electrolyte in the chamber, in which theset of fins acts as an electrode and defines therebetween separatechannels for conducting the upward current of electrolyte in thechamber, a wall of said channels being formed by a porous web bearingthe membrane allowing ions to pass therethrough, wherein the fins have aheight between 5 and 10 mm, and the distance separating two adjacentfins is between 5 and 10 mm to ensure a velocity of the electrolyteadjacent to the fins of at least 0.01 m/s,wherein the membrane isarranged between the anode and the metal strip to be plated so that saidmembrane separates the cell into a cathodic space adjacent to the metalstrip and an anodic space defined by the chamber of the enclosure andforms a separation between the cathodic space of the cell and thechamber defined by the enclosure of the electrode, wherein in saidprocess a first circuit of the first electrolyte in the cathodic spaceand a second circuit of the second electrolyte in the chamber arecreated, the membrane preventing the passage of gases formed at theanode into the first circuit of the first electrolyte and the passage ofthe salt of the plating metal from the cathodic space into the secondcircuit of the second electrolyte in the chamber.
 2. Process for theelectrochemical plating of a metal strip according to claim 1 with ironcompounds, in which the first electrolyte is a sulfuric electrolyteenriched in iron and zinc sulfate and in which the second electrolyte isdevoid of metal and consists of water and sulfuric acid, wherein ananion exchange membrane is arranged between the anode and the metalstrip to be plated, the membrane allowing the transfer of charge only bythe transfer of SO₄ ⁼ ions into the anode chamber and preventing thepassage of the metal salts, so that the second electrolyte devoid ofmetal is supplementary enriched in sulfuric acid, and the oxygen formedat the insoluble anode is discharged from the anode chamber and thepassage of oxygen into the cathodic space is prevented by means of theanion exchange membrane.
 3. Process according to claim 2 in which thepart of electrolyte in excess which is formed, the nature of which issimilar to that of the anode chamber, is conveyed to the cathodicelectrolyte circuit through a dissolving station.
 4. Process for theelectrochemical plating of a metal strip according to claim 1 with ironcompounds, in which the first electrolyte is a chloride electrolyteenriched in iron and zinc chloride, and in which the second electrolyteis devoid of metal and consists of water and chlorhydric acid, in whichan anion exchange membrane is arranged between the anode and the metalstrip to be plated, wherein the membrane allows the passage of chlorinein the anode chamber but prevents the passage of the metals salts, sothat the second electrolyte is not enriched in metal salts, and thechloride transformed into the second electrolyte flow devoid of metal ofthe anode chamber is removed and the passage of chlorine in the cathodicspace is prevented by the anion exchange membrane.
 5. Process for theelectrochemical plating of a metal strip according to claim 1 with ironcompounds, in which the first electrolyte is a sulfuric electrolyteenriched in iron and zinc sulfate and in which the second electrolyte isdevoid of metal and consists of water and sulfuric acid, in which acation exchange membrane is arranged between the anode and the metalstrip to be plated, wherein the membrane prevents the transfer of acidsfrom the cathodic space into the anode chamber and allows the transferof charge by the transfer of hydrogen ions from the anode chamber intothe cathodic space, and the oxygen formed at the anode is removed fromthe second electrolyte devoid of metal of the anode chamber and thepassage of oxygen in the cathodic space is prevented by the cationexchange membrane.
 6. Process according to claim 5, in which the part ofelectrolyte in excess which is formed, the nature of which is similar tothat of the anode chamber, is conveyed to the cathodic electrolytecircuit through a dissolving station.
 7. Process for the electrochemicalplating of a metal strip according to claim 1 with iron compounds, inwhich the first electrolyte is a chloride electrolyte enriched in ironand zinc chloride and in which the second electrolyte is devoid of metaland consists of water and chlorhydric acid, in which a cation exchangemembrane is arranged between the anode and the metal strip to be plated,wherein the membrane prevents the passage of acids and salts from thecathodic space into the anode chamber and allows the transfer of chargeby the transfer of hydrogen ions from the anode chamber into thecathodic space, and the gases formed at the anode are removed from theanode chamber with the second electrolyte devoid of iron containingchlorhydric acid and the passage of gases formed in the cathodic spaceis prevented by the cation exchange membrane.
 8. Process according toclaim 1 for plating a metal strip with iron compound, wherein to replacethe iron deposited on the metal strip, an amount of elemental ironcorresponding to the deposited amount is added to the first electrolytewhich flows through the cathodic space.
 9. Vertically oriented electrodefor electrolytic cell, said electrode comprising a set of vertical finshaving a bottom and a top, said set of fins being located within anenclosure defining a chamber, wherein a vertical wall of said enclosureis formed by a membrane allowing ions to pass therethrough, saidenclosure having a feeding passage for feeding an electrolyte into thechamber at the bottom of the set of fins and an outlet passage forevacuating electrolyte from the chamber at the top of the set of finscreating an upward current of electrolyte in the chamber, in which theset of fins acts as an electrode and defines therebetween separatechannels for conducting the upward current of electrolyte in thechamber, a wall of said channels being formed by a porous web bearingthe membrane allowing ions to pass therethrough, wherein the fins have aheight between 5 and 10 mm, and the distance separating two adjacentfins is between 5 and 10 mm to ensure a velocity of the electrolyteadjacent to the fins of at least 0.01 m/s.
 10. Electrode according toclaim 9, in which the enclosure has a third opening at the top of theset of fins for discharging gases outside the chamber.
 11. Electrodeaccording to claim 9 in which the membrane is an anionic membrane. 12.Electrode according to claim 9 in which the membrane is provided with aprotective layer.
 13. Electrode according to claim 9 in which a poroussupport contacts the membrane and acts as supporting means for at leastone part thereof.
 14. Electrode according to claim 13, in which thesupport is selected from the group consisting of a perforated component,a porous web and a trellis.
 15. Electrode according to claim 13, inwhich the support has a first face contacting the membrane and a secondface opposite that adjacent to the membrane, wherein the second face isprovided with a layer acting as an electrode.
 16. Electrode according toclaim 13, in which the membrane rests on a support acting as anelectrode, said support is provided with an insulating layer on its faceadjacent to the membrane.
 17. A method of using an electrode comprisingproviding an electrode according to claim 1 in an electrolytic cell. 18.Electrode according to claim 9, in which the membrane is a cationicmembrane.
 19. Vertically oriented electrode for electrolytic cell, saidelectrode comprising:(a) a first set of vertical fins having a bottomand a top, said first set of fins being located within a first enclosuredefining a first chamber, wherein a vertical wall of said enclosure isformed by a membrane allowing ions to pass therethrough, said firstenclosure having a feeding passage for feeding an electrolyte into thefirst chamber at the bottom of the first set of fins and an outletpassage for evacuating electrolyte from the chamber at the top of thefirst set of fins creating an upward current of electrolyte in the firstchamber, in which the first set of fins acts as an electrode and definestherebetween separate channels for conducting the upward current ofelectrolyte in the chamber, a wall of said channels being formed by aporous web bearing the membrane allowing ions to pass therethrough, saidporous web acting as an electrode, wherein the fins have a heightbetween 5 and 10 mm and a distance separating two adjacent fins between5 and 10 mm ensuring a velocity of the electrolyte adjacent to the finsof at least 0.1 m/s; (b) a second set of vertical fins having a bottomand a top, said second set of fins being located within a secondenclosure defining a second chamber, said second enclosure being abovethe first enclosure, wherein a vertical wall of said second enclosure isformed by a membrane allowing ions to pass therethrough, said secondenclosure having a feeding passage for feeding an electrolyte into thesecond chamber at the bottom of the second set of fins and an outletpassage for evacuating electrolyte from the second chamber at the top ofthe second set of fins creating an upward current of electrolyte in thesecond chamber, in which the second set of fins acts as an electrode anddefines therebetween separate channels for conducting the upward currentof electrolyte in the second chamber, a wall of said channels beingformed by a porous web bearing the membrane allowing ions to passtherethrough, said porous web acting as an electrode, wherein the finshave a height between 5 and 10 mm and a distance separating two adjacentfins between 5 and 10 mm ensuring a velocity of the electrolyte adjacentto the fins of at least 0.1 m/s; (c) a cross member linking the firstenclosure with the second enclosure, said cross member having passagesextending between the outlet passage of the first enclosure and thefeeding passage of the second enclosure, where electrolyte flowing outfrom the first chamber flows into the second chamber, and (d) a beadingextending between the first enclosure and the second enclosure, saidbeading extending beyond the vertical walls of the first and secondenclosures formed by a membrane.
 20. Electrode according to claim 19, inwhich the second enclosure has a third opening above the top of thesecond set of fins for discharging gases outside the second chamber. 21.Process for the electrochemical plating of a metal strip with a metal ina cell provided with an insoluble anode and in which the metal stripacts as a cathode, in which a first electrolyte containing a salt of aplating metal is recycled between a cathodic metal strip to be platedand an insoluble anode, in which the insoluble anode is an electrode,wherein said electrode comprises:(a) a first set of vertical fins havinga bottom and a top, said first set of fins being located within a firstenclosure defining a first chamber, wherein a vertical wall of saidenclosure is formed by a membrane allowing ions to pass therethrough,said first enclosure having a feeding passage for feeding a secondelectrolyte into the first chamber at the bottom of the first set offins and an outlet passage for evacuating second electrolyte from thechamber at the top of the first set of fins creating an upward currentof second electrolyte in the first chamber, in which the first set offins acts as an electrode and defines therebetween separate channels forconducting the upward current of second electrolyte in the chamber, awall of the said channels being formed by a porous web bearing themembrane allowing ions to pass therethrough, said porous web acting asan electrode, wherein the fins have a height between 5 and 10 mm and adistance separating two adjacent fins between 5 and 10 mm ensuring avelocity of the second electrolyte adjacent to the fins of at least 0.1m/s; (b) a second set of vertical fins having a bottom and a top, saidsecond set of fins being located within a second enclosure defining asecond chamber, said second enclosure being above the first enclosure,wherein a vertical wall of said second enclosure is formed by a membraneallowing ions to pass therethrough, said second enclosure having afeeding passage for feeding an electrolyte into the second chamber atthe bottom of the second set of fins and an outlet passage forevacuating electrolyte from the second chamber at the top of the secondset of fins creating an upward current of electrolyte in the secondchamber, in which the second set of fins acts as an electrode anddefines therebetween separate channels for conducting the upward currentof electrolyte in the second chamber, a wall of said channels beingformed by a porous web bearing the membrane allowing ions to passtherethrough, said porous web acting as an electrode, wherein the finshave a height between 5 and 10 mm and a distance separating two adjacentfins between 5 and 10 mm ensuring a velocity of the electrolyte adjacentto the fins of at least 0.1 m/s; (c) a cross member linking the firstenclosure with the second enclosure, said cross member having passagesextending between the outlet passage of the first enclosure and thefeeding passage of the second enclosure, where the second electrolyteflowing out from the first chamber flows into the second chamber, and(d) a beading extending between the first enclosure and the secondenclosure, said beading extending beyond the vertical walls of the firstand second enclosures formed by a membrane, wherein the membrane isarranged between the anode and the metal strip to be plated so that saidmembrane separates the cell into a cathodic space adjacent to the metalstrip and an anodic space defined by the chambers of the enclosures andforms a separation between the cathodic space of the cell and thechambers defined by the enclosures of the electrode, wherein in saidprocess a first circuit of the first electrolyte in the cathodic spaceand a second circuit of the second electrolyte in the chambers arecreated, the membrane preventing the passage of gases formed at theanode into the first circuit of the first electrolyte and the passage ofthe salt of the plating metal from the cathodic space into the secondcircuit of the second electrolyte in the chambers.
 22. Process for theelectrochemical plating of a metal strip according to claim 21 with ironcompounds, in which the first electrolyte is a sulfuric electrolyteenriched in iron and zinc sulfate and in which the second electrolyte isdevoid of metal and consists of water and sulfuric acid, wherein ananion exchange membrane is arranged between the anode and the metalstrip to be plated, the membrane allowing the transfer of charge only bythe transfer of SO₄ ⁻² ions into the anodic space and preventing passageof metal salts, so that the second electrolyte devoid of metal issupplementally enriched in sulfuric acid, and oxygen formed at theinsoluble anode is discharged from the anodic space and passage ofoxygen into the cathodic space is prevented by means of the anionexchange membrane.
 23. Process for the electrochemical plating of ametal strip according to claim 21 with iron compounds, in which thefirst electrolyte is a chloride electrolyte enriched in iron and zincchloride, and in which the second electrolyte is devoid of metal andconsists of water and chlorhydric acid, in which an anion exchangemembrane is arranged between the anode and the metal strip to be plated,wherein the membrane allows passage of chlorine into the anodic spacebut prevents passage of metal salts, so that the second electrolyte isnot enriched in metal salts, and chloride transformed into the secondelectrolyte flow devoid of metal in the anodic space is removed andpassage of chlorine into the cathodic space is prevented by the anionexchange membrane.
 24. Process for the electrochemical plating of ametal strip according to claim 21 with iron compounds, in which thefirst electrolyte is a sulfuric electrolyte enriched in iron and zincsulfate and in which the second electrolyte is devoid of metal andconsists of water and sulfuric acid, in which a cation exchange membraneis arranged between the anode and the metal strip to be plated, whereinthe membrane prevents transfer of acids from the cathodic space into theanodic space and allows transfer of charge by transfer of hydrogen ionsfrom the anodic space into the cathodic space, and oxygen formed at theanode is removed from the second electrolyte devoid of metal in theanodic space and the passage of oxygen into the cathodic space isprevented by the cation exchange membrane.
 25. Process for theelectrochemical plating of a metal strip according to claim 21 with ironcompounds, in which the first electrolyte is a chloride electrolyteenriched in iron and zinc chloride and in which the second electrolyteis devoid of metal and consists of water and chlorhydric acid, in whicha cation exchange membrane is arranged between the anode and the metalstrip to be plated, wherein the membrane prevents passage of acids andsalts from the cathodic space into the anodic space and allows transferof charge by transfer of hydrogen ions from the anodic space into thecathodic space, and gases formed at the anode are removed from theanodic space with the second electrolyte devoid of iron containingchlorhydric acid and passage of gases formed in the cathodic space isprevented by the cation exchange membrane.